Installing Python 3.6 on any version of Blender
It’s to do with setting the PYTHONPATH in blender somewhere.  Two min and I will see if I can find it on my phone...

Edit I can’t find the exact answer on my phone. Search for “change python version blender” and you will get the answer, it’s been asked a few times but not for 2.8 as far as I can see.

Also the answer varies depending on OS.

Here it is... Change Python version used by blender